Printed circuit boards are well known industrial products and are used for a wide variety of commercial and consumer electronic applications such as in appliances, radios, machines, etc. Basically, the board contains a desired circuit configuration in the form of electrically conductive material "printed" onto the board. Boards having circuits on each face of the board are electrically interconnected between certain areas on the opposite faces by through-holes, which are drilled or punched in the desired configuration, and the walls coated with an electrically conductive material.
There are essentially two methods for preparing printed circuit boards (PCB's) one being termed the "subtractive" method and the other termed the "additive" method. Both methods are well known and are described in U.S. Pat. No. 4,233,344, which patent is hereby incorporated by reference. Basically, in the additive system, the starting board is comprised of plastic with no metal foil, and the metal circuit is then built up upon the nonconductive substrate in the desired pattern. In one of the subtractive methods, a nonconductive substrate, such as epoxy bonded fiberglass, has adhered to two sides thereof a metal cladding or laminate, most often copper. Holes are drilled through the copper laminate board exposing the plastic. It is then deburred, chemically cleaned and rinsed. The board is then treated for plating the through-holes by contacting with a dilute acid solution, dipped into a catalyst, most commonly an acidic palladium-tin catalyst, to activate the plastic for electroless deposits, rinsed in water, treated with an accelerator to activate the catalyst metal, again rinsed and immersed into an electroless plating bath to plate all catalyzed surfaces of the board including formation of a conductive coating on the inside of the through-hole to electrically connect the two metal (copper) sides. An etch resist is then applied over the unwanted copper areas in the circuit pattern desired. The board is then cleaned, electroplated with copper and coated with Sn/Pb. The etch resist is then removed with an appropriate solution (either solvent or alkaline solution) to expose the underlying foil and electroless copper thereon and this copper is removed by etching, thereby providing the desired circuit.
Regardless of the method for making printed circuit boards, the nonconductive portions of the substrate must be activated prior to metallization by either electroless or electrolytic metal plating. Unfortunately, it is not commercially feasible to treat only the nonconductive portions of the board and, as a result, the entire board is immersed or dipped into all the treating baths, including both the catalyst solution and the electroless plating bath.
To overcome the apparent waste of copper and other problems associated with prior art processes, newer methods electrolessly plate at the end of the PCB manufacturing process. These methods typically catalyze the drilled panel first and then utilize an etch resist (dry film type) over the desired circuit pattern, including tenting of the through holes, with the unwanted copper then being etched using ammoniacal copper chloride o ferric chloride. The etch resist is then removed and the board electrolessly plated, with the only portions plated being the circuit lines and the through-holes.
The use of an organic solvent soluble etch resist normally does not present a problem, unfortunately, however, removal of the etch resist using, e.g., alkaline solutions, removes the catalyst from the through-holes making the metal plating ineffective. This is a particularly serious problem when an environmentally preferred aqueous soluble etch resist is used since the commonly used KOH stripping solution readily removes the catalyst adsorbed on the through-hole wall.
It is an object of the subject invention to provide a method for treating a catalyzed nonconductive substrate to enhance its resistance to compositions used in the plating process. It is a further object of the present invention to provide an efficient and effective method for preparing a printed circuit board containing through-holes. Another object of the present invention is to provide printed circuit boards prepared by the novel method of the invention. These and other objects of the present invention will become readily apparent from the following detailed description.